Spectral-domain OCT measurements in obesity: A systematic review and meta-analysis

Background Previous studies proposed possible applications of spectral-domain optical coherence tomography (SD-OCT) measurements in prognosticating pathologies observed in overweight/obesity, including ocular, vascular, and neurologic consequences. Therefore, we conducted a systematic review and meta-analysis to investigate the changes in the in SD-OCT measurements of the patients with higher body mass index (BMI) compared to normal weight individuals. Materials and methods We conducted a systematic search on PubMed, Scopus, and Embase. The search results underwent two-phase title/abstract and full-text screenings. We then analyzed SD-OCT measurements differences in patients with high BMI and controls, and performed meta-regression, sub-group analysis, quality assessment, and publication bias assessment. The measurements included macular thickness, cup to disc ratio, ganglion cell-inner plexiform layer (GC-IPL) and its sub-sectors, RNFL and peripapillary RNFL (pRNFL) and their sub-layers, and choroidal thickness and its sub-sectors. Results 19 studies were included in this meta-analysis accounting for 1813 individuals, 989 cases and 824 controls. There was an overall trend towards decreased thickness in high BMI patients, but only two measurements reached statistical significance: temporal retinal nerve fiber layer (RNFL) (Standardized mean difference (SMD): -0.33, 95% confidence interval (CI): -0.53 to -0.14, p<0.01) and the choroidal region 1.0 mm nasal to fovea (SMD: -0.38, 95% CI: -0.60 to -0.16, p<0.01). Conclusion Some ocular layers are thinner in patients with higher BMI than the controls. These SD-OCT measurements might correlate with adverse events related to increased body weight and have prognostic abilities. As SD-OCT is a robust, rapid and non-invasive tool, future guidelines and studies are needed to evaluate the possibility of their integration into care of the patients with obesity.

titles and abstracts to inclusion criteria. The ineligible records in this step were excluded and the eligible ones entered another phase where we examined their full-texts to select the eligible studies. Discrepancies were solved through discussion with a third author (S.M.). This study was registered in the International Prospective Register of Systematic Reviews (PROSPERO) with the code of CRD42021238501. using the formula previously described by Wan et al. [22], and included the studies in metaanalysis. However, we also performed sensitivity analysis omitting the studies that reported median instead of mean, only including those in the sensitivity analysis that originally reported mean (SD) values.
We used Higgins I 2 test to assess the heterogeneity of the studies reporting the SD-OCT measurements. Variables with I 2 values above 40% was analyzed using random-effect models, while the others underwent fixed-effect analysis.
Egger's test and funnel plots helped us assess the potential publication bias for each of the measurements. We used trim and fill strategy to overcome the potential publication bias in the variables with asymmetric funnel plots that reached significant Egger's test results.
We performed a meta-regression analysis on several variables to detect the potential confounders that affect the results, including study methods, participant baseline characteristics, some ocular parameters, and several variables related to metabolic syndrome and obesity. We also performed sub-group analyses based on the patient's age group (children or adult), method of eye selection (single eye, both eyes, or mixed eyes), matching status of the studies, and SD-OCT model. Childhood was defined as younger than 18 years old, and adulthood as older than this age. An earlier study on SD-OCT measurements in Alzheimer's disease found differences between various SD-OCT devices (e.g. Zeiss Cirrus HD, Heidelberg Spectralis, etc.) and changes in thicknesses of some layers between cases and controls [17]. This study encouraged us to provide these analyses taking into account the type of SD-OCT device utilized. Meta-regression and sub-group analyses were only performed on the variables with sufficient number of studies.

Macular thickness
In total, 5 studies examined macular thickness, responsible for 272 cases and 197 controls [9,20,[26][27][28]. No significant difference was observed between the groups (SMD: -0.20, 95% CI: -0.61 to 0.22, p = 0.36, S1 Fig). We also performed a sensitivity analysis to examine what was the result of the studies that reported mean (SD), as Baran et al. data were converted to mean (SD) as mentioned in the methods. The remaining four studies also did not show a significant difference (SMD: -0.08, 95% CI: -0.53 to 0.37, p = 0.74).

Choroidal thickness
A total of 12 studies measured various parameters of choroidal thickness, accounting for 634 cases and 569 controls [9, 14, 20, 25- Table 2 illustrates the analyses for all the SD-OCT measurements.
We also performed a sensitivity analysis for sub-foveal region thickness including the studies that reported their data as mean (SD), and found no significant difference between subfoveal thickness of cases and controls (SMD: -0.20, 95% CI: -0.46 to 0.06, p = 0.13).
Insignificant variables for both measurements included study sample size, year of study, mean age, matching status for age/sex, eye measurement method (single-eye vs. both eyes), OCT models, mean intraocular pressure of controls, mean Spherical equivalent of cases, and body mass index (BMI) of cases. Insignificant variables for either of average pRNFL and subfoveal thicknesses-significant or unavailable data for the other-were the male proportion, mean intraocular pressure of cases, mean Spherical equivalent of controls, ocular axial length of cases and controls, BMI of controls, BMISDS of controls, triglyceride level of cases and controls, cholesterol level of cases, low density lipoprotein (LDL) level of cases, high density lipoprotein (HDL) of cases, HOMA-IR of controls, systolic blood pressure (SBP) of cases and controls, and diastolic blood pressure (DBP) of cases and controls ( Table 3).

Subgroup analysis
We performed subgroup analysis on the average pRNFL and sub-foveal choroidal region thicknesses, as they had sufficient number of studies. Both parameters had borderline significance (p = 0.05) as mentioned above. Table 4 presents the details of the sub-groups analyses for these two parameters.
3.8.1. Average pRNFL. We found that the average pRNFL thickness was only statistically lower in higher BMI children (SMD: -0.33, 95% CI: -0.63 to -0.03, p = 0.03, S12 Fig  Significant or borderline p-values of 0.05 or less for overall effect and I 2 >40% are marked in bold ( � ).

Publication bias
We performed Egger's test for all the analyzes and found no publication bias in any of them, except for the macular thickness and average choroidal thickness that had significant Egger's test and asymmetrical funnel plot. Trim and fill was executed for these two parameters. The analysis for average choroidal thickness remained the same before and after trim and fill. Trim and fill method calculated corrected numbers for macular thickness that confirmed the observed insignificant relationship between the BMI and macular thickness (SMD: -0.08, 95% CI: -0.49 to 0.33, p = 0.72) (S2 Table).

Quality assessment
Most of the studies scored acceptable to good scores based on the NOS (mean: 5.5, range: 3-8, out of 8). S1 Table illustrates the quality score details based on their scores in selection, comparability, and exposure. The least favorable component was their comparability, as many studies did not match their cases and controls with each other regarding age or sex. However, most of studies scored well in the selection and comparability categories.

Discussion
In this meta-analysis, we found that most of the SD-OCT measurements trended towards decreased thickness, but only two measurements were significantly thinner in patients with Aging can be another contributing factor and we can divide it into actual and accelerated aging mechanisms. RNFL thickness decreases after 50 years of age, by 2.2 μm per decade [45,46]. The prevalence of obesity increases with aging until the age of 50-59 when it reaches its peak prevalence [47]. Therefore, studies unadjusted for age can produce lower RNFL thicknesses for patients with obesity partly due to their older age, although such hypothesis can only be applicable for 50-59 years when obesity prevalence increases and RNFL thickness decreases. However, this might not be the case as we found no relationship between mean age and average pRNFL thickness in meta-regression, and age-matched and non-age-matched subgroup analyses did not produce different results. Furthermore, our sub-group analysis yielded significant results only for children and not the adults. This shows that the significantly lower average pRNFL thickness came from the significance of the studies on children, and not the adults. Therefore, the unmatched studies on adults could not confound the significantly lower average pRNFL thickness due to the confounder of higher obesity prevalence and lower average pRNFL thickness in people older than 50. On the other hand, obesity can accelerate the epigenetic aging process [48]. Therefore, it is possible that RNFL thinning can be started earlier in the life of the patients with obesity due to their premature aging. Premature aging may also contribute the significantly lower average pRNFL thickness in children.
Increased IOP might be another factor of decreased RNFL in obesity. Several previous studies established a relationship between obesity and increased IOP [26, 28, 49]. One study considered their statistically significant increase too small to be of clinical significance [26]. Glaucoma and increased IOP might play roles in RNFL thinning [50][51][52]. However, a large population-based study of 11,030 healthy eyes found that IOP was not associated with any RNFL measurements; but the authors attributed this finding to the lack of data on the pretreatment pressures of the individuals on IOP-lowering therapies [53]. A recent populationbased study demonstrated that after adjusting for the potential confounders, each 1 mmHg increase in the IOP correlated with a 0.05 μm/year faster decrease in RNFL thickness [54]. We did not observe such correlations between IOP of cases and controls and average pRNFL thickness. However, other specific sub-sectors could have such correlations that we could not analyze due to the lack of sufficient studies.
Obesity induces a low-grade inflammatory status in the body [55]. Increased mitochondrial and peroxysomal fatty acid oxidation leads to higher oxidative stress, thereby inducing systemic inflammation [56]. Excessive lipid mass increases secretion of pro-inflammatory cytokines and adipokines such as leptin, resistin, IL-6, tumor necrosis factor-alpha (TNF-α), and plasminogen activator inhibitor-1 (PAI-I), and suppresses the anti-inflammatory cytokines such as adiponectin [26, 57,58]. Markers of endothelial activation and dysfunction are also increased in individuals with obesity [10]. Leptin, for instance, is a hormone secreted by adipose tissue that regulates energy intake and expenditure and serves as an acute-phase reactant [59]. It has similar functional and structural similarities to IL-6 and play roles in differentiation, proliferation, and survival of numerous cells including endothelial cells, lymphocytes, and neutrophils, reducing vasodilator response by nitric oxide (NO), and increasing oxidative stress [57,60]. The adipokines then provoke the production of reactive oxygen species (ROS) and promote the level of oxidative stress [61]. Endothelial dysfunction in obesity leads to decreased vasodilator molecules, such as NO, and increased levels of vasoconstrictor compounds, again stimulating inflammation and ROS production [57,62]. The ongoing inflammation and increased ROS promote ganglion cell death and axonal loss following axonal injury via both apoptosis and necrosis pathways [56]. Therefore, inflammatory pathways in obesity lead to a decreased RNFL, in fact, they are also responsible for some of the other abovementioned mechanisms, including neurological disorders, aging, and increased IOP [11,25,28].
The last mechanism involves other metabolic disorders and hypertension coexisting with obesity. Although we did not observe a relationship between lipid profile and average pRNFL thickness in the meta-regression, HOMA-IR and BMISDS reduced its thickness. Zarei et al. previously established the relationship between higher number of metabolic abnormalities and lower RNFL thickness [48]. Chronic inflammation and increased oxidative stress stimulate insulin resistance and also deprive retinal cells of neurotrophic factors essential for their survival, including insulin, leukemia inhibitory factor, and ciliary neurotrophic factor [63]. Baran et al. also observed a negative correlation between BMISDS and waist-hip ratio (WHR)-representing central obesity-with decreased RNFL thickness [26]. Hypertension was another factor that we could not examine in the meta-regression, but was previously introduced as a possible factor responsible for RNFL thinning. Hypertension seemed to affect the temporal RNFL quadrant more than the others that can partly explain why we observed significant RNFL thinning only in this quadrant [64]. Overall, we should holistically take these proposed mechanisms for RNFL thinning into account, as they are related to each other and not isolated pathways.
Male proportion, mean spherical equivalent, BMISDS, and HOMA-IR had correlations with average pRNFL in meta-regression. Previous literature also suggested such correlations. Epic-Norfolk study found thinner RNFL in older patients, male sex, higher BMI (only in males), short axial length, and the history of cataract surgery, but not an increased IOP [53]. Refractive status also affected RNFL measurements in previous studies [65,66]. As mentioned above, insulin is considered a neurotrophic factor essential for retinal cell survival, and therefore, insulin resistance measured by HOMA-IR contributes to lower average pRNFL thickness [10,63]. The correlation between BMISDS and decreased RNFL thickness was observed in other studies [12,26]. We did not find correlations between age and average pRNFL thickness. This could be related to the included studies, as most of them recruited children. Children and adolescents of various ages have similar RNFL thickness [67,68], and the decrease in RNFL thickness starts around 50 years old [46]. Therefore, we expect that our sample population could not show such correlation, even if it existed.
We observed that sub-foveal choroidal thickness, the choroidal region 1.0 mm nasal to fovea, and the choroidal region 1.0 mm temporal to fovea were significantly thinner in patients with higher BMI. Choroid is the vascular layer of the eye and 65-85% of the ocular blood flow passes through its vascular plexus [35]. Higher BMI is associated with structural changes in the macrovascular and microvascular system [19,69]. Therefore, the observed changes in the choroidal vasculature might be responsible for the observed decrease in the choroidal thickness [36].
Obesity provokes changes in the molecular environment of choroid and subsequently causes changes in the diameter of the retinal vasculature. Choroid is the only layer of the eye that its vessels are regulated through sympathetic and parasympathetic mechanisms, the former reducing the choroidal blood flow with noradrenaline and the later increasing it through NO pathway [70]. Higher BMI increases the levels of vasoconstrictor molecules such as angiotensin-II and endothelin-1, and reduces the levels of vasodilators such as NO [71,72]. NO regulates ocular perfusion to the choroidal tissue, optic nerve, and retina [73]. Retinal-originated dopamine, which secrets choroidal-originated NO, is also decreased in individuals with obesity [74,75]. The decreased NO and dopamine levels in the eye can increase choroidal vascular resistance, reduce the blood flow, and finally decrease the choroidal thickness [75][76][77]. Furthermore, several studies found that obese children and adults have decreased retinal arteriolar caliber and dilated retinal venules, probably due to the dysregulations in vasodilator/vasoconstrictor balance [18,19]. Abnormal blood flow might result from the disrupted arterial blood supply, autonomic vasoregulation, and endothelial injury, further damaging the choroid and altering its thickness [78].
We demonstrated that IOP correlated with sub-foveal choroidal thickness. As mentioned earlier, individuals with obesity have higher IOP [26,28,49]. Several mechanisms are linked to the higher IOP, including: 1) Increased retrobulbar fat diminishes aqueous outflow and results in a higher IOP [79], 2) Alterations in plasma levels of ghrelin and leptin can increase IOP and induce glaucoma [80,81]. Hyperleptinemia in patients with obesity may increase the oxidative stress and spark the pathological changes causing higher IOP [81,82]. Ghrelin is a hormone with antioxidant and anti-inflammatory effects and may affect the anterior and posterior segments of the eye [83]. Katsanos et al. found lower ghrelin levels in the anterior chamber of the glaucoma group compared to the controls [80]. Ghrelin also possessed neuro-protective properties on the retina of the glaucomatous rats [84]. Therefore, decreased ghrelin levels in patients with obesity may trigger higher IOP [83,85]. 3) Elevated blood viscosity might increase episcleral venous pressure, hampering aqueous outflow and leading to a higher IOP [86], 4) Concurrent hyperglycemia can osmotically shift fluid into the eyes and increasing its pressure [87], 5) Altered microcirculation is another factor that increases IOP in patients with higher BMI [19]. IOP elevates in obesity partly because of the decreased levels of vasodilators, such as NO, and increased vasoconstrictors, including angiotensin-II and endothelin-I [71,72].
Decreased choroidal thickness have been reported in several other conditions coexisting with obesity, including diabetes [88], hypertension [89], and obstructive sleep apnea-hypopnea syndrome [90]. Nevertheless, we did not find correlations between systolic and diastolic blood pressures and sub-foveal choroidal thickness. These comorbidities may be another factor affecting the choroidal thickness in patients with obesity. Fortunately, there are strategies to counter the effect of obesity on the choroidal thickness and restore ocular blood flow. Studies found that training [91] and bariatric surgery [92] improved the decreased retinal arteriole to venule ratio in obesity. Furthermore, choroidal thickness increased in the bariatric surgery group compared to those scheduled for conservative management [92].
Choroidal thickness increases in children as they get older [93,94], and decreases after 60 years of age by 5.40 μm for each year [95]. Yumusak et al. also proposed female sex as a predictor of diminished choroidal thickness [14], but we did not observe such relationship between age and sex in sub-foveal thickness. Mapelli et al. found that children with decreased axial length had thinner choroidal volume [94], again a finding that our study did not reproduce based on sub-foveal thickness meta-regression.
We did not observe any significant changes in the GC-IPL layer thickness. This finding contrasted our expectations, as increased oxidative stress and IOP can induce ganglion cell death and reduce the GC-IPL thickness [56,96]. Levin et al. suggested that ganglion cell death is a delayed process following the axonal injury, and many ganglion cells may survive several days after the axonal injury [96]. Factors favoring ganglion cell survival may play role in this observation [10]. Therefore, these insults may decrease GC-IPL thickness at a later stage of life. As all the included studies on GC-IPL thickness in this meta-analysis comprised children, we hypothesize that GC-IPL thickness might be decreased in the adults with higher BMI. Furthermore, low number of studies with small populations reporting these parameters might also be another factor influencing this lack of significant results, as average, nasal-superior, and temporal-inferior GC-IPL thicknesses had p-values between 0.05 and 0.1.
We did not observe a significant difference in cup to disc ratio between patients with high BMI and controls, although the p-value approached the threshold of significance (p = 0.08). Three large population-based studies found lower cup to disc ratio in patients with higher BMI [97][98][99]. However, none of them included pediatric patients, and two of them included patients older than 40 [97] and 45 years [98]. On the other hand, 3/5 of our studies were conducted in children [26,30,31] and the other two on adults [27, 28] also did not have similar compositions to the population-based studies. Therefore, differences in participants might justify the conflicting findings between this study and the aforementioned population-based studies.
Understanding the SD-OCT measurements not only impact the ocular health of the individuals with obesity, but also may have screening and prognosticating benefits for them. For instance, choroidal thickness is an important factor as it can predict several systemic and ocular complications related to metabolic syndrome, such as hypertension, diabetic retinopathy, age-related macular degeneration, and myopia [14,18,27,89]. It also provides valuable noninvasive information on the important vascular changes in obesity, including cardiovascular diseases [18][19][20]. Changes in retinal vasculature is also known to be a marker of systemic microvascular changes in obesity [20,34]. Retinal arteriole to venule ratio is considered an indicator of preclinical alterations in coronary and cerebral microcirculation [92]. Ganglion cell layer (GCL) thickness also correlates well with the visual field analysis and the risk of several neuro-ophthalmic pathologies in the future, including Parkinson's and Alzheimer's disease, which are higher among individuals with obesity [17,100]. Therefore, SD-OCT measurements may provide precious prognostic information on the health status of the individuals with obesity, and we encourage conducting future studies and developing guidelines to detect the patients that may benefit the most from prediction ability of SD-OCT measurements. This fast, non-invasive, and accurate three-dimensional imaging may serve as an adjunct risk-stratification tool among other clinical and laboratory studies in patients with higher BMI. Nevertheless, various follow-up studies are required to carefully further assess the relationships between SD-OCT measurements and obesity, the suitability and cost-effectiveness of SD-OCT in the clinical settings, and also address the limitations of the previous studies, such as their lack of reference to/matching of various important parameters (e.g. age, sex, concurrent medications used, etc.). Furthermore, differences in results in children and adults should also be discovered in larger and more robust studies to find the best markers based on each age group.
To the best of our knowledge, this study is the first meta-analysis investigating the changes in choroidal and retinal thickness related to increased body weight. However, several limitations exist in our study. Many ocular parameters had few numbers of studies with small populations, and therefore, they may have not reached statistical significance due to this small sample size. Furthermore, studies on some parameters only consisted of specific age groups, hiding potential relationship between higher BMI and those parameters in other age groups. Another limitation arises from the countries of studies, where Turkish articles account for most of the studies. As thickness of some ocular layers vary with ethnicity, including RNFL and pRNFL [101,102], the included studies may only represent a group of ethnicities based on their country of origin. Also, several possibly important parameters were not reported and matched for by the included studies, including the other metabolic profiles of the patients and the concurrent medications used by them. Furthermore, we could not analyze the parameters based on the severity of obesity, while some earlier studies demonstrated the degree of obesity might alter some of the findings [14].

Conclusion
Obesity is associated with diminished thickness in several SD-OCT measurements through various direct and indirect mechanisms. Although most of the layers face decreases in thickness, only two measurements reached statistical significance: temporal RNFL and the choroidal region 1.0 mm nasal to fovea. The changes in the thickness of SD-OCT measurements might serve as a risk-stratification tool to predict several ocular and systemic complications of obesity, as earlier pieces of research found associations between these complications and changes in the thicknesses of various ocular layers. Future studies and guidelines need to verify our findings, address the limitations of the studies, and establish a base for integrating SD-OCT into the care of the patients with high BMI.